Nitrate Leaching in Brazilian Cropping Systems without Nitrogen Fertilization.

Nitrate (NO₃^-) leaching is one of the main routes of nitrogen (N) loss from the soil-plant system, and in general, leaching increases with N rate in N-fertilized cropping systems. Less is known about N leaching in systems without N fertilization. In this study we aimed to evaluate N leaching in cropping systems under no-till with no N fertilization and different N additions by cover crops. This experiment has been ongoing since 2006 in Botucatu, São Paulo, Brazil. Ruzigrass (Brachiaria ruziziensis) and grain sorghum (Sorghum bicolor) were grown in the fall/winter, either alone or intercropped and pearl millet (Pennisetum glaucum), forage sorghum (Sorghum bicolor) and sunn hemp (Crotalaria juncea) were grown in the spring, in sub-plots. In the summer, soybean (Glycine max (L.) Merrill) was planted in all sub-plots. Nitrate leaching was calculated based on NO₃^- concentrations in soil solution samples collected at 80-cm depth and on the estimated drainage below that layer, and was determined for one year (from May 2012 to April 2013). Residue production from each crop was also determined. Nitrogen losses via NO₃^- leaching reached up to 23.1 kg ha^-1 yr^-1 and should be considered in the N balance for systems without added N fertilizer. The magnitude of these losses is determined by the amount of N present in the soil and plants and by the relative carbon (C) and N concentrations of the plants. Sunn hemp and grain sorghum resulted in higher NO₃^- leaching because there was more N in the systems where these species were present. Pearl millet had a higher C/N ratio, delaying residue mineralization and depressing soil N levels, which resulted in less leaching. Hence, it is important to alternate biological N-fixing species with plants with high N accumulation capacity and high C/N ratio to decrease N leaching.